Typically you would see this message if the performance counters have not been registered correctly on the server. When SQLmonitor fails to find a performance counter, it usually prints the category or object to the error pop-up along with the error
message you posted. You can use this to look in perfmon and see if the counters are there or not.

If you have missing perfmon counters for your instance, you can follow this suport article to rebuild counters in perfmon:

Were you doing this on a default instance, or a named instance ?? Also for remote monitoring, you need to ensure that the remote registy service is running, and that any firewall is configured to allow the montoring tool to get this data. The
windows account you are using to also needs to have the rights to add perfmon counters.

SQL Live Monitor translates the value from perfmon in sections into Days (D) Hours (H) Minues (M) Seconds (S). Since 300 seconds (the value from perfmon) is 5 minutes, you should expect to see a value that is more than 5 mins.

The sample interval will depend on the granulaity you want from the tool. 1 second would be the same as viewing perfmon in real time, 15 seconds if the default for permon captureing data to a file. Typically if you are capturing over time, then
you are looking for trends in the daya, so 5 secs to 15 secs should be fine. If however you want to see specific spikes at finer intervals then drop the sample interval down to 1 second.

The tool samples the data every seconds to display on the main form, so there is very little additional overhead in writning it out to aCSV at the same time.